Alumina-zirconia-silicon carbide-magnesia cutting tools

ABSTRACT

A metalcutting insert is provided with an alumina based ceramic composition composed of (in volume percent, v/o): about 1.5 to 17.5 silicon carbide whiskers; about 5 to 17.5 zirconia; residue of a magnesium oxide addition added in an amount effective to enhance the metalcutting lifetime of the cutting edge of the insert; and an alumina based matrix in which the silicon carbide whiskers, zirconia and residue of magnesium oxide are substantially homogeneously distributed. At least 4 v/o of the alumina based ceramic composition is tetragonal zirconia.

BACKGROUND OF THE INVENTION

The present invention relates to alumina based ceramic cutting toolscontaining zirconia and silicon carbide. It especially relates toceramic cutting tools useful in the high speed, rough machining of softsteels and ductile or malleable cast irons.

In the past, zirconia has been added in specified amounts to selectedalumina-silicon carbide whisker reinforced compositions described inU.S. Pat. Nos. 4,534,345 and 4,507,224 to provide enhanced fracturetoughness and/or flexural strength (see: U.S. Pat. Nos. 4,657,877 and4,749,667; Japanese Patent Publication No. Sho 62-265182; Clausen et al,"Whisker-Reinforced Oxide Ceramics," Journal de Physique Colloque Cl,Supplement au No. 2, Tome 47, February 1986, Pages Cl-693 to Cl-702;Becher et al, "Toughening of Ceramics by Whisker Reinforcement,"Fracture Mechanics of Ceramics 7, ed. by Bradt et al, Plenum Press, NewYork (1986), Pages 61-73).

It has been indicated that the zirconia should be in the monoclinicand/or (metastable) tetragonal phase to obtain improved fracturetoughness and/or flexural strength. It has been further indicated thatthe metastable tetragonal phase is obtained by reducing zirconiaparticle size or through the use of a cubic zirconia stabilizationpromoter such as yttria, calcia, magnesia and the rare earth oxides inamounts below that required to fully stabilize the cubic zirconia phase.

Cutting tools composed of a variety of compositions containing alumina,zirconia and silicon carbide whiskers with or without other additiveshave been proposed (see: European Patent Application No. 86107916.8(published Jan. 21, 1987 as No. 0208910); U.S. Pat. No. 4,749,667;"Multitoughening Ceramic," Techno Japan, Vol. 19, No. 10, October 1986,Page 78; and European Patent Application No. 86301597.0 published Sept.17, 1986, as No. 0194811).

Where an indication is given as to the material being cut by thesetools, these tools have been applied to the machining of cast irons,hardened steels and nickel based superalloys. These are all materialsthat have relatively low reactivity with the silicon carbide in thecutting insert at the high temperatures encountered during metalcutting.None of the foregoing documents teaches or suggests that, formetalcutting inserts for use in the high speed roughing of soft steels,cutting performance can be significantly improved by controlling thealumina based ceramic composition to within the combination of criticalranges now discovered by the applicants for zirconia, magnesia, siliconcarbide and tetragonal zirconia contents.

SUMMARY OF THE INVENTION

Applicants have surprisingly discovered that alumina based ceramiccutting inserts containing (in volume percent, v/o) about 1.5 to 17.5v/o silicon carbide whiskers, about 5 to 17.5 v/o zirconia, the residueof a magnesium oxide or other magnesium-oxygen compound addition, and atleast 4 v/o tetragonal zirconia, have excellent cutting edge lifetimeswhen engaged in the high speed rough machining of soft steel. It hasbeen surprisingly found that, despite the finding that minor magnesiaadditions act to reduce the amount of tetragonal (i.e., metastabletetragonal) zirconia at room temperature, an effective amount of thisaddition has a significant positive affect on cutting edge lifetime inthe high speed rough turning of soft steels, such as AISI (American Ironand Steel Institute) 1045 steel.

In accordance with the present invention, it has been surprisingly foundthat the magnesia addition can be added to the present composition inamounts effective to preferably provide the cutting edge with a lifetimeof at least 12 minutes when turning AISI 1045 steel, having a Brinellhardness number in the range of 190 to 200, under the conditions of 1000surface feet/minute, 0.025 inch/revolution and 0.100 inch depth of cut.These magnesia additions, more preferably, can produce cutting edgelifetimes of at least 15, and most preferably, of at least 20 minutes.It is preferred that magnesia be added in amounts of about 0.03 to 3v/o; more preferably, about 0.03 to 2.0 v/o; and most preferably, about0.04 to 1.0 v/o.

Preferably, the alumina based ceramic composition according to thepresent invention contains about 2.5 to 15 v/o and, more preferably,about 2.5 to 10.5 v/o silicon carbide whiskers.

The zirconia content according to the present invention is preferably7.5 to 17.5 v/o and, more preferably, about 10 to 15 v/o. In accordancewith the present invention, a significant fraction of the zirconia(preferably at least 22 volume percent) is in the form of tetragonalzirconia and must form at least 4 v/o of the composition. Preferably, atleast 6 v/o; more preferably, at least 7 v/o; and most preferably, atleast 8 v/o of the composition is in the form of tetragonal zirconia.

These and other aspects of the present invention will become moreapparent upon review of the detailed description of the presentinvention in conjunction with the figures briefly described below:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of an embodiment of a square cutting insertin accordance with the present invention.

FIG. 2 shows a graph of the volume percent tetragonal zirconia in thecomposite as a function of the volume percent of magnesia, or yttria,addition.

DETAILED DESCRIPTION OF THE INVENTION

In the present invention, as shown in FIG. 1, a metalcutting insert 10,preferably of the indexable type, is provided having a rake face 30, aflank face 50 and cutting edge 70 at the juncture of the rake and theflank faces. The cutting edge 70 is preferably in a chamfered condition(e.g., K-land) as shown in FIG. 1. The metalcutting insert is composedof an alumina based ceramic composition containing: about 1.5 to 17.5v/o silicon carbide whiskers; about 5 to 17.5 v/o zirconia; and aresidue of a magnesium oxide or other magnesium-oxygen compound additionadded in an amount effective to enhance the metalcutting lifetime of thecutting edge when engaged in the high speed rough turning of a soft ironbase alloy, such as AISI 1045 steel having a hardness of about 190 to200 BHN (Brinell Hardness Number). The silicon carbide whiskers,zirconia and residue of magnesia are substantially homogeneouslydistributed in an alumina based matrix.

The silicon carbide whiskers are present at a level of at least about1.5 v/o to assure minimal levels of cutting edge lifetime improvement.More preferably, silicon carbide whiskers are present at about 2.5 v/oor more. The silicon carbide whisker content preferably should notexceed about 17.5 v/o of the composition. We believe that siliconcarbide whisker contents above this value result in a significantdecrease in the cutting edge lifetime during the high speed roughturning of soft steels. Therefore, to further maximize cutting edgelifetime during high speed roughing of steels, such as AISI 1045, it ispreferred that the maximum content of silicon carbide whiskers be heldat or below about 15 v/o and, more preferably, at or below about 10.5v/o of the alumina based ceramic composition.

The silicon carbide whiskers utilized herein may be any of thecommercially available brands which have been used in the past inalumina based metalcutting inserts for machining nickel basesuperalloys.

While less preferred, silicon carbide particles of a generally equiaxedshape or platelet shape may be substituted for part of the siliconcarbide whiskers in this invention.

The zirconia content is in the range of about 5 to 17.5 v/o of theceramic composition. Zirconia contents outside of this range arebelieved to provide compositions having reduced cutting edge lifetimeduring the high speed roughing of AISI 1045 steel. Preferably, tomaximize cutting edge lifetime, the zirconia content should be withinthe range of about 7.5 to 17.5 v/o and, more preferably, about 10 to 15v/o of the composition. While we believe that the concentration oftetragonal zirconia should be maximized for best cutting performance, itis equally, if not more, important in our opinion that as much of thetetragonal zirconia as possible, present at room temperature, beavailable for transformation toughening at, or as near as possible to,the temperatures encountered at the cutting edge during machining. Itis, therefore, critical to the present invention that magnesia bepresent in certain small, but effective, amounts which have been foundcritical to the maximization of cutting edge lifetime. In accordancewith the present invention, at least about 22 volume percent of thezirconia present, and at least 4 v/o of the ceramic composition, must betetragonal zirconia. Preferably, the tetragonal zirconia forms at leastabout 6 v/o of the composition, more preferably, at least about 7 v/o ofthe composition, and most preferably, at least about 8.0 v/o of thecomposition.

Magnesia additions, despite the fact that they decrease the amount oftetragonal zirconia observed at room temperature, are preferably addedin the range of about 0.03 to 3 v/o of the composition, more preferably,about 0.03 to 2 v/o; most preferably, about 0.04 to 1.0 v/o. Magnesiamay be blended in with the alumina or zirconia just prior to compactpressing, or it may be preblended or prealloyed with the alumina orzirconia. Preblending of the magnesia powder is preferred since it isbelieved that the preblended magnesia is more effective in producinghigh temperature metastable tetragonal zirconia, thus allowing a smalleramount of magnesia to be added and minimizing the deleterious effects ofhigh magnesia additions (e.g., lower melting point glass and Mg--Al--Oformations). Equivalent amounts of other magnesium-oxygen compounds,such as magnesium carbonate, which may require an additional processingstep such as calcination to produce magnesia, may be substituted for allor part of the magnesia addition. After sintering of the blendedcompositions, the magnesia addition may not exist as a separate phasebut as a residue. This residue may include, for example,magnesia-alumina solid solution, magnesium aluminate, a magnesiazirconia solid solution and/or a glass, for example, formed with silicondioxide impurities which may have existed as a thin coating on thesilicon carbide whiskers.

The remainder of the ceramic composition is essentially alumina andpreferably entirely alumina except for impurities. In all cases, thepresent alumina based ceramic composition contains at least 40 v/oalumina and, preferably, at least about 50 v/o alumina.

Titanium carbide, as whiskers and/or substantially equiaxialparticulate, may be added in an amount of about 2 to 35 v/o of thecomposition and, preferably of about 10 to 30 v/o. Titanium carbide hasa higher thermal expansion coefficient than alumina. It is, therefore,believed that titanium carbide additions should allow more tetragonalzirconia to be retained at room temperature. Titanium carbide whiskersmay be manufactured and harvested by the methods described in A. Kato etal, "Growth Rate of Titanium Carbide Whiskers in Chemical VaporDeposition," J. Cryst. Growth, 37 (1977), Pages 293-300; and N. Tamariet al, "Catalytic Effects of Various Metals and Refractory Oxides on theGrowth of TiC Whiskers by Chemical Vapor Deposition," J. Cryst. Growth.46 (1979), Pages 221-237. Titanium carbide whiskers and theirincorporation and use in alumina based cutting inserts are disclosed inMehrotra et al U.S. patent application Ser. No. 056,091, filed May 28,1987, and assigned to Kennametal Inc. now U.S. Pat. No. 4,852,999.

The alumina powders utilized herein should be high purity alumina(i.e., >99% pure) such as produced by ALCOA (e.g., grade A16SG), byCeralox (e.g., grade HPA - 0.5 with or without magnesia) or by ReynoldsChemicals (grade RC-HP or RC-HP-DBM).

Yttria, calcia, the rare earth oxides, and other compounds which have,through a reduction in the tetragonal to monoclinic transformationtemperature, an adverse affect on cutting edge lifetime are preferablypresent only as impurities, if present at all.

The foregoing material, in accordance with the present invention, may bemilled, blended, and densified at high temperature to produce at least98%, and preferably, at least 99% dense alumina based ceramiccompositions having an alumina based matrix, which is preferablyentirely alumina, in which the silicon carbide, magnesia residue fromthe magnesia addition, zirconia and titanium carbide, if any, are atleast substantially homogeneously distributed. Hot pressing or sinteringtemperatures are preferably held below 1700° C., and more preferably,below 1650° C. and, most preferably, below about 1600° C. to minimizezirconia particle growth and thereby maximize the tetragonal zirconiaphase present in the final product. The average zirconia particle sizein the cutting insert should not exceed about 5 microns, preferablyshould not exceed 2 microns, and more preferably, should not exceed 1micron. However, the average zirconia particle size should be largeenough to allow most tetragonal zirconia to transform to monocliniczirconia during use. This minimum size will depend upon the ceramiccomposition and is presently undetermined.

While not wishing to be bound by any particular theory, applicants offerthe following explanation of the present invention. In alumina-siliconcarbide whisker-zirconia compositions, the amount of metastabletetragonal zirconia that can be obtained at room temperature can beincreased by a reduction in zirconia particle size or the addition ofthe so-called cubic stabilizing agents, such as yttria, calcia and/orthe rare earth oxides. (see: Stevens, "An Introduction toZirconia--Zirconia and Zirconia Ceramics," Magnesium Elektron Pub. No.113, Magnesium Elektron Ltd., England (1986)). While the literaturecommonly includes magnesia among the foregoing list of stabilizingagents, applicants have found that, when magnesia is added in the amountof about 0.03 to 3 v/o to the present compositions, magnesia decreasesthe amount of tetragonal zirconia present at room temperature. Whenyttria is added to zirconia, it tends to stabilize the tetragonal andcubic phases of zirconia to a lower temperature. All the aforementionedstabilizing agents, and most impurities, except for magnesia, affectZrO₂ in similar ways (i.e., they reduce the temperature at which thetetragonal zirconia phase is stable). At room temperature, some of thezirconia may be present as metastable tetragonal zirconia. Under theaction of tensile stress, this tetragonal zirconia may becomemonoclinic, giving rise to transformation toughening. However, as thetemperature increases, tetragonal zirconia becomes stable and,therefore, unavailable for transformation to the monoclinic crystalstructure. Thus, any impurity or additive, such as yttria, whichstabilizes tetragonal zirconia at lower temperatures, is unsuitable formetalcutting applications since the cutting tip temperature may rise to1000° to 1200° C. in high speed machining. Therefore, in accordance withthe theory of the present invention, additives which raise themonoclinic to tetragonal transformation temperature of the zirconia arerequired for high temperature transformation toughening. We have foundthat there are only two oxide additives, MgO and HfO₂, which raise thetransformation temperature. Thus, we believe that by keeping thezirconia particle size small, a large proportion of the zirconia can beretained as metastable tetragonal zirconia at the high temperature ofmetalcutting by alloying the zirconia with magnesia or hafnia. It is ourbelief that this helps to obtain enhanced tool lives in metalcuttingoperations. It should be noted that zirconia normally contains up toabout 2 w/o (weight percent) hafnia as an impurity.

The significant positive impact that magnesia additions have onmetalcutting performance is more clearly indicated by the followingexamples which are purely illustrative of the present invention.

Six compositions were prepared (Table 1) with the nominal compositionsbeing Al₂ O₃ - 10 v/o SiC_(w) (silicon carbide whiskers) - 10 v/o ZrO₂.Small additions of Y₂ O₃ and MgO were made. In the case of Mix No. 6,about 0.05 w/o (approximately 0.06 v/o) MgO had already been blendedwith Al₂ O₃ by the powder manufacturer. This provided Mix No. 6 with amagnesia content of about 0.04 v/o. Fifty gram batches of these powderswere prepared by first blending Al₂ O₃ and ZrO₂ (and stabilizingadditives, if any) slurries (propanol) in a jar mill using Al₂ O₃cycloids for one hour. Sonicated SiC_(w) slurry was then added, and thewhole mix was blended for one hour. Al₂ O₃ and ZrO₂ slurries hadpreviously been milled to obtain mean particle sizes of 0.5 to 0.6 μmand 0.6 to 0.8 μm, respectively (corresponding specific surface areasmeasured by BET were 10 to 14 m² /g and 20 to 40 m² /g, respectively).Then, the mix was pan dried, screened through 100 mesh screen, and hotpressed in a one inch diameter graphite die at the temperatures andpressures shown below in Table 1 for one hour in argon. The resultingbillets were more than 99% dense, and were cut, ground and polished formeasurement of physical and mechanical properties. The billets were alsocut and ground to produce indexable cutting inserts for metalcuttingtests.

                  TABLE 1                                                         ______________________________________                                        COMPOSITIONS                                                                  ______________________________________                                        Nominal Composition: Al.sub.2 O.sub.3 - 10 v/o SiC.sub.w - 10 v/o             ZrO.sub.2                                                                     Al.sub.2 O.sub.3 :                                                                     Alcoa A16SG                                                          SiC.sub.w :                                                                            Tokai Carbon Co. (Tokyo, Japan)                                               TOKAWHISKER (TOKAMAX) Grade 1                                                 (0.3-1.0 μm diameter; 20-50 μm                                          length)                                                              ZrO.sub.2 :                                                                            Zircar - unstabilized                                                MgO:     Fisher Scientific Corp. - Reagent                                             Grade BET = 40.4 m.sup.2 /g                                          Y.sub.2 O.sub.3                                                                        MolyCorp, BET = 15.4 m.sup.2 /g                                      ______________________________________                                        Hot Pressing                                                                  Mix No.                                                                              Temp. (°C.)                                                                       Pressure (psi)                                                                           Composition                                      ______________________________________                                        1.     1650       4000       Nominal                                          2.     1650       4500       Nominal + 1 v/o Y.sub.2 O.sub.3                  3.     1625       4500       Nominal + 1 v/o MgO                              4.     1600       4500       Nominal + 1 v/o Y.sub.2 O.sub.3 +-   1 v/o                                    MgO                                              5.     1625       4500       Nominal, except that                                                          Al.sub.2 O.sub.3 used was Ceralox                                             Grade HPA - 0.5 (0.5                                                          to 0.7 μm median                                                           particle size)                                   6.     1625       4500       Nominal, except that                                                          Al.sub.2 O.sub.3 used was Ceralox                                             Grade HPA - 0.5 with                                                          MgO (0.05 w/o)                                   ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        PROPERTIES                                                                                        Tetragonal ZrO.sub.2.sup.2                                Mix  RA        Fracture Toughness                                                                           as v/o as v/o of                                No.  Hardness  K.sub.IC (E&C).sup.1 (MPa m.sup.1/2)                                                         of ZrO.sub.2                                                                         Composite                                ______________________________________                                        1    93.3      5.59           84     8.4                                      2    93.3      4.97           100    10                                       3    93.6      5.96           70     7                                        4    93.6      4.63           98     9.8                                      5    93.9      5.88           84     8.4                                      6    93.8      5.88           76     7.6                                      ______________________________________                                         .sup.1 Evans and Charles, "Fracture Toughness Determination by                Indentation," J. American Ceramic Society, Vol. 59, No. 7-8, Pages 371,       372, using an 18.5 kg load.                                                   .sup.2 As measured by xray diffraction of a polished surface. The             remainder of the zirconia is assumed to be monoclinic zirconia. Cubic         zirconia, which may be present in minor amounts, is included within the       tetragonal zirconia estimate.                                            

Physical and mechanical properties of the hot pressed composites areshown in Table 2.

The Porter-Heuer (Porter et al, J. American Ceramic Society, Vol. 62,No. 5-6 (1979), pages 298-305) formula was modified and used to estimatethe fraction of monoclinic ZrO₂ (Vm) from peak intensities of the 111reflection of the monoclinic ZrO₂ (Im(111)), and 111 reflection of thetetragonal ZrO₂ (It(111): ##EQU1## where Vt is the fraction of ZrO₂which is tetragonal ZrO₂.

The estimated volume fraction of tetragonal ZrO₂ in the whole compositeis (vt):

    vt=Vt vz                                                   (3)

where vz is the volume fraction of the total ZrO₂ added in the mix. Theabove relationship assumes that ZrO₂ substantially remains unchangedduring consolidation except for the phase transformations discussedabove.

FIG. 2 shows the effect of the various additives on the amount oftetragonal zirconia in the composites. It can clearly be seen thatmagnesia additions lower the amount of tetragonal zirconia (curve 1),whereas, yttria additions increase the amount of tetragonal zirconia atroom temperature (curve 2).

                  TABLE 3                                                         ______________________________________                                        TURNING AISI 1045* STEEL (192-200 BHN)                                        Insert   Cutting Edge Life &                                                                            Average Life                                        Material Failure Mode     (minutes)                                           ______________________________________                                        Mix 1    14. BK      14.7 BK  14.4                                            Mix 2     8. BK      12.6 BK  10.3                                            Mix 3    15.9 BK     29.2 FW  22.6                                            Mix 4     1.7 FW      7.7 FW   4.7                                            Mix 5    17. FW       7.5 BK  12.3                                            Mix 6    22.9 DN     32.9 FW  27.9                                            ______________________________________                                               Test Conditions:                                                              1000 sfm (surface feet/minute)                                                0.025 ipr (inch/revolution)                                                   0.100 inch doc (depth of cut)                                                 SNGN-453T (American National Standard                                           Designation in accordance with ANSI B212.4                                    1986) indexable cutting insert style                                          (cutting edge preparation:                                                    0.008 inch × 20° K-land)                                       15° lead angle (side cutting edge angle)                               -5° side rake angle                                                    -5° back rake angle                                                    no coolant                                                                    Cutting Edge Life Criteria:                                                   FW - .015" uniform flank wear                                                 MW - .030" concentrated flank wear                                            CR - .004" crater wear                                                        DN - .030" depth of cut notch                                                 CH - .030" concentrated wear or chipping                                      BK - breakage                                                          ______________________________________                                         *AISI 1045 is equivalent to Unified Numbering System (UNS) Designation        G10450.                                                                  

Indexable insert cutting edge lifetimes in the high speed roughing of apremachined AISI 1045 steel are shown in Table 3. It can be clearly seenthat a significant improvement in cutting edge lifetime is achieved bythe addition of magnesia, whereas, a decrease in tool life occurs whenyttria is added despite the high level of tetragonal zirconia present inthe yttria containing compositions.

                  TABLE 4                                                         ______________________________________                                        COMPOSITIONS                                                                  ______________________________________                                        Nominal Composition: Al.sub.2 O.sub.3 - 5 v/o SiC.sub.w - 10 v/o              ZrO.sub.2                                                                     Al.sub.2 O.sub.3 :                                                                     Ceralox-HPA-.5 without MgO                                           SiC.sub.w :                                                                            Tokai Grade 1                                                        ZrO.sub.2 :                                                                            Magnesium Elektron (SC15)-                                                    unstabilized (.5-.6 μm particle                                            size BET 5-8 m.sup.2 /g)                                             ______________________________________                                        Hot Pressing                                                                  Mix                                                                           No.   Temp. °C.                                                                         Pressure (psi)                                                                           Composition                                       ______________________________________                                        7.    1535° C.                                                                          5000 psi   Nominal + .05 v/o MgO                             8.    1550° C.                                                                          5000 psi   Nominal + .25 v/o MgO                             9.    1550° C.                                                                          5000 psi   Nominal + .50 v/o MgO                             10.   1550° C.                                                                          5000 psi   Nominal + 1.0 v/o MgO                             11.   1550° C.                                                                          5000 psi   Nominal + 3.0 v/o MgO                             ______________________________________                                    

A second series of mixes, 7 to 11 shown in Table 4, were made to furtherdemonstrate the effect that the level of the magnesia has on tetragonalzirconia content and cutting edge lifetime. All samples were processedand hot pressed essentially as described with respect to the samplesproduced from Mixes 1 to 6.

The physical and mechanical properties of the materials are reported inTable 5. It can clearly be seen that the tetragonal zirconia contentagain clearly decreases with increasing amounts of magnesia addition.This affect is also shown in FIG. 1, curve 3. It can be seen that Mixes7 to 11 have a higher tetragonal zirconia content than that found in thematerials represented by curve 1. The affect is believed to be

due to the lower SiC_(w) content (5 v/o versus 10 v/o) used in thesecond group of mixes. Applicants have observed that generally, as SiCwhisker content increases, the amount of tetragonal zirconia decreasesfor a given zirconia content, and everything else being held constant.

                  TABLE 5                                                         ______________________________________                                        PROPERTIES                                                                                                         Tetragonal                                                                    ZrO.sub.2 as                             Mix    V/o     RA       Fracture Toughness                                                                         v/o of                                   No.    MgO     Hardness K.sub.IC (E&C) (MPa m.sup.1/2)                                                             Composite                                ______________________________________                                        7      0.05    93.5     5.57         8.5                                      8      0.25    93.4     5.00         8.1                                      9      0.50    93.3     5.06         8.0                                      10     1.0     93.4     4.75         7.6                                      11     3.0     93.5     4.98         7.3                                      ______________________________________                                    

Indexable insert cutting edge lifetimes in the high speed roughing ofAISI 1045 steel are shown in Table 6.

                  TABLE 6                                                         ______________________________________                                        TURNING AISI 1045 STEEL (197-199 BHN)                                         Insert   Cutting Edge Life   Average Life                                     Material & Failure Mode      (Minutes)                                        ______________________________________                                        Mix  7   19      bk      5     ch/bk 12                                       Mix  8   12.6    dn/ch   1     bk    6.8                                      Mix  9   12.3    dn      14.1  dn    13.2                                     Mix 10   9.7     bk      7.1   dn    8.4                                      Mix 11   7.0     bk      6.8   dn    6.9                                      ______________________________________                                    

The test procedures and conditions and cutting edge life criteria usedto generate the data shown in Tables 5 and 6 were the same as thatdescribed for Tables 2 and 3.

In another example in accordance with the present invention, acomposition containing Al₂ O₃ -2.5 v/o SiC_(w) -10 v/o ZrO₂ -1.05 v/oMgO was made. A fifty gram batch of this composition was prepared byfirst blending an Al₂ O₃ (Ceralax Grade HPA-0.5 with MgO (0.05 w/o)),ZrO₂ (Magnesium Elektron SC15) and MgO (Fisher Reagent Grade) slurry(propanol) in a jar mill using Al₂ O₃ cycloids for one hour. A SonicatedSiC_(w) (Tokai Grade 1) slurry was then added and the whole mix wasblended for one hour. The Al₂ O₃ and ZrO₂ containing slurries hadpreviously been milled to obtain mean particle size of about 0.5-0.7 μmand 0.5 to 0.6 μm, respectively. Then the mix was pan dried, screenedthrough a 100 mesh screen and isostatically compacted at 30,000 psi atroom temperature. Pieces were then cut from the resulting cold compactedblank and sintered at 1700 degrees Centigrade for one hour in oneatmosphere argon followed by hot isostatic pressing at 1600 degreesCentigrade for one hour in 17,000 psi argon. The resulting samples weregreater than 99 percent dense (i.e., fully dense). As described in theprior examples, samples were then prepared for physical and mechanicaltesting and ground into indexable cutting inserts. It was determinedthat the samples contained about 6.6 v/o tetragonal zirconia. It isestimated that material processed in this manner has a zirconia particlesize of about 5 μm or less. Cutting inserts of the style described inTable 3 were tested under the conditions used in Table 3. Cutting edgelife times of 14.4 (DN failure) and 18.9 (FW & CH failure) minutes wereobtained.

It is believed that cutting edge lifetime may be extended or made moreuniform if the cutting edge is honed and/or the insert surface is lappedor polished to remove surface material containing a higher percentage ofmonoclinic and a lower percentage of tetragonal zirconia than ischaracteristic of the bulk of the material (i.e., a polished surface).It is known that grinding stresses create a surface layer in which aportion of the metastable tetragonal zirconia has been transformed tomonoclinic zirconia. It is preferred that at least those surface areasof the insert which will encounter high temperatures during use have themaximum amount of tetragonal zirconia available for high temperaturetransformation.

All patents, patent applications and documents referred to herein arehereby incorporated by reference.

Other embodiments of the invention will be apparent to those skilled inthe art from a consideration of this specification or practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with the true scope and spiritof the invention being indicated by the following claims.

What is claimed is:
 1. A metalcutting insert comprising:a rake face; aflank face; a cutting edge at a juncture of said rake face and saidflank face; said cutting insert having an alumina based ceramiccomposition consisting essentially of:about 1.5 to 17.5 v/o siliconcarbide whiskers; about 5 to 17.5 v/o zirconia; a residue of a magnesiaaddition added in the amount of about 0.03 to 3 v/o; alumina formingessentially the remainder of said composition; wherein said siliconcarbide whiskers, said zirconia and said residue of said magnesiaaddition are substantially homogeneously dispersed in a matrix formed ofsaid alumina; and wherein at least about 4.0 v/o of said ceramiccomposition is tetragonal zirconia.
 2. The metalcutting insert accordingto claim 1 wherein at least about 6.0 v/o of said composition istetragonal zirconia.
 3. The metalcutting insert according to claim 1wherein at least 7 v/o of said composition is tetragonal zirconia. 4.The metalcutting insert according to claim 1 wherein said residue ofsaid magnesia addition is about 0.03 to 2 v/o.
 5. The metalcuttinginsert according to claim 1 wherein said residue of said magnesiaaddition is about 0.04 to 1 v/o.
 6. The metalcutting insert according toclaim 1 wherein said silicon carbide whiskers is about 2.5 to 15 v/o. 7.The metalcutting insert according to claim 1 wherein said siliconcarbide whiskers is about 2.5 to 10.5 v/o.
 8. The metalcutting insertaccording to claim 1 wherein said zirconia is about 7.5 to 17.5 v/o. 9.The metalcutting insert according to claim 1 wherein said zirconia isabout 10 to 15 v/o.
 10. The metalcutting insert according to claim 1wherein said zirconia has an average particle size of less than 2microns.
 11. The metalcutting insert according to claim 1 wherein saidzirconia has an average particle size of less than or equal to onemicron.
 12. A metalcutting insert comprising:a rake face; a flank face;a cutting edge at a juncture of said rake face and said flank face; saidcutting insert having an alumina based ceramic compositioncomprising:about 1.5-17.5 v/o silicon carbide whiskers; about 5 to 17.5v/o ZrO₂ ; a residue of a magnesia addition added in an amount effectiveto provide said cutting edge with a lifetime of at least 12 minutes whenturning AISI 1045 steel having a Brinell hardness number in the range of190 to 200, under the conditions of 1000 surface feet/minute, 0.025inch/revolution and 0.100 inch depth of cut; an aluminum based whereinsaid silicon carbide whiskers, said zirconia and said residue of saidmagnesium oxide addition are substantially homogeneously distributed;and wherein tetragonal zirconia forms at least 4 v/o of said aluminabased ceramic composition.
 13. The metalcutting insert according toclaim 12 wherein said lifetime is at least about 15 minutes.
 14. Themetalcutting insert according to claim 12 wherein said lifetime is atleast about 20 minutes.
 15. The metalcutting insert according to claim12 containing at least 6 v/o tetragonal zirconia.
 16. The metalcuttinginsert according to claim 12 containing at least 7 v/o tetragonalzirconia.
 17. The metalcutting insert according to claim 12 containingat least 8 v/o tetragonal zirconia.
 18. The metalcutting insertaccording to claim 12 wherein said alumina based matrix is entirelyalumina except for impurities.
 19. The metalcutting insert according toclaim 12 wherein said zirconia has an average particle size no greaterthan 5 μm.
 20. The metalcutting insert according to claim 12 whereinsaid zirconia has an average particle size of less than 2 μm.
 21. Themetalcutting insert according to claim 1 wherein said silicon carbidewhiskers are present at a level of about 1.5 v/o.
 22. The metalcuttinginsert according to claim 5 wherein said silicon carbide whiskers arepresent at a level of about 1.5 v/o.
 23. The metalcutting insertaccording to claim 8 wherein said silicon carbide whiskers are presentat a level of about 1.5 v/o.
 24. The metalcutting insert according toclaim 9 wherein said silicon carbide whiskers are present at a level ofabout 1.5 v/o.
 25. The metalcutting insert according to claim 12 whereinsaid silicon carbide whiskers are present at a level of about 1.5 v/o.26. The metalcutting insert according to claim 13 wherein said siliconcarbide whiskers are present at a level of about 1.5 v/o.
 27. Themetalcutting insert according to claim 14 wherein said silicon carbidewhiskers are prevent at a level of about 1.5 v/o.
 28. The metalcuttinginsert according to claim 18 wherein said silicon carbide whiskers arepresent at a level of about 1.5 v/o.
 29. The metalcutting insertaccording to claim 18 wherein said silicon carbide whiskers are presentat a level of about 2.5 v/o.
 30. The metalcutting insert according toclaim 5 wherein said silicon carbide whiskers are present at a level ofabout 2.5 v/o.
 31. The metalcutting insert according to claim 8 whereinsaid silicon carbide whiskers are present at a level of about 2.5 v/o.32. The metalcutting insert according to claim 9 wherein said siliconcarbide whiskers are present at a level of about 2.5 v/o.
 33. Themetalcutting insert according to claim 12 wherein said silicon carbidewhiskers are present at a level of about 2.5 v/o.
 34. The metalcuttinginsert according to claim 13 wherein said silicon carbide whiskers arepresent at a level of about 2.5 v/o.
 35. The metalcutting insertaccording to claim 18 wherein said silicon carbide whiskers are presentat a level of about 2.5 v/o.
 36. The metalcutting insert according toclaim 12 wherein said silicon carbide whiskers are present at a level ofabout 5 v/o.
 37. The metalcutting insert according to claim 18 whereinsaid silicon carbide whiskers are present at a level of about 5 v/o.